Data are commonly read from and written to spinning magnetic disks. The disks commonly have a series of closely spaced, concentric data tracks. A slider, including a data head, is mounted to a support arm which biases the slider towards the data surface of the disk. The slider is typically an air bearing slider and is supported just above the spinning data surface of the disk by a thin film of air. An actuator incremently moves the support arm so the slider is indexed to be registered with the desired data track.
To accommodate small manufacturing offsets in the slider surface relative to the data surface as well as small tilt variations caused by the movement of the data surface, the slider is typically mounted to the support arm through a gimbal arrangement. This gimballed mounting of the slider to the support arm permits the slider to pivot about a tangentially-directed axis in a radially-oriented plane, such pivotal movement commonly called roll, and about a radially-oriented axis in a tangentially-oriented plane, such movement commonly called pitch. This movement is illustrated in FIG. 1.
One problem associated with the manufacture of hard disk drives is that in an effort to get more information on the same size disk, the data tracks are spaced closer together. Presently, hard disks typically have about 4000 to 10,000 tracks per inch (about 1575 to 3900 tracks per cm); track densities of up to about 25,000 tracks per inch (about 9800 tracks per cm) are contemplated and even higher densities are expected in the future. These higher track densities and higher rotational speeds combine to increase the problem of misregistration of the data head with the data track. One article discussing this disk misregistration problem states: "Current high performance disk drives have track densities approaching 6,000 TPI, or 166 micro inches between tracks. The head positioning servo system must hold the read/write element within 12% of the center of the track which means that relative radial displacement between the head and disk must be no more than 20 micro inches. At these high track densities, effects which were formally negligible now contribute significantly to track misregistration (TMR) in the 3.5 inch disk drives." ("The Effect of Disk Platter Resonances on Track Misregistration in 3.5 Inch Disk Drives," IEEE Transaction Magnetics, Vol. 32, No. 3, May 1996, pp. 1762-1766.) With the track densities heading towards the 10,000 to 25,000 tracks per inch range, track misregistration problems become even more severe.
Disks have natural vibration modes, each characterized by a particular resonant frequency. At these frequencies the disk movement will be amplified by the resonant response to even small excitations. The driving excitations can be produced by imperfect bearings and motors as well as by aerodynamic instabilities in the air flow across the disk surface, particularly at the higher rotation rates. Although these vibrations are most easily characterized by the movement of the outer regions of the disk in a direction perpendicular to the surface, this motion necessarily must produce a tilting of the disk surface since the disks are clamped at the inside radius. It can be shown that the neutral plane of the disk, the plane at the middle of the thickness, does not move radially to any significant degree. Due to the finite thickness of the disk, the tilting requires that the slider surface and the disk surface must move in opposite radial directions depending on the sense of the tilt. In latest drives these radial motions are large enough and at a high enough frequency as to challenge the ability of the drive servos to keep the heads centered on the data tracks.